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Thread: New to cs... Evaluative metering

  1. #21

    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by mengwei11 View Post
    Thanks. Sort of (I think) understand that. Just didn't understand the way the camera behaves wrt evaluative and spot, and metering vs focus point.
    Don't mix up metering with focus.
    Focus points are just that, points where the cam can focus at i.e. either center or any point you/cam chooses.

    Metering: cam calculates exposure based on what it sees in the viewfinder.
    Evaluative/matrix tries to have a uniform exposure across the entire scene.
    Center-weighted gives more weight to the center.
    Spot calculates exposure based on a 5-10% area of the scene, usually around the center focus point - though on some cameras the "spot" can be shifted to other focus points.

  2. #22

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    Quote Originally Posted by cks2k2

    Don't mix up metering with focus.
    Focus points are just that, points where the cam can focus at i.e. either center or any point you/cam chooses.

    Metering: cam calculates exposure based on what it sees in the viewfinder.
    Evaluative/matrix tries to have a uniform exposure across the entire scene.
    Center-weighted gives more weight to the center.
    Spot calculates exposure based on a 5-10% area of the scene, usually around the center focus point - though on some cameras the "spot" can be shifted to other focus points.
    That's where it gets confusing. If back to my example a tree occupying one quarter of the bottom right frame. I centre focus on tree, lock and recompose such that the tree is at bottom corner. Some one said that when I focus lock (semi depress shutter button) , the exposure (aperture and shutter speed is locked), now what does the camera meter? For that tree in the bottom left corner since I focus locked ( and hence exposure) or the subsequent re composed view. Assuming evaluative

  3. #23
    Senior Member Diavonex's Avatar
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    Default Re: New to cs... Evaluative metering

    mengwei11, I don't know what camera you are using.

    Go to your camera manual and spend some time reading:

    1. Metering mode

    2. Focusing mode

    3. AE Lock

    If you don't get item 1 and 2 sorted out, you'll have plenty of problems getting correct exposure and focus.
    Last edited by Diavonex; 16th February 2011 at 11:45 PM.

  4. #24

    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by mengwei11 View Post
    That's where it gets confusing. If back to my example a tree occupying one quarter of the bottom right frame. I centre focus on tree, lock and recompose such that the tree is at bottom corner. Some one said that when I focus lock (semi depress shutter button) , the exposure (aperture and shutter speed is locked), now what does the camera meter? For that tree in the bottom left corner since I focus locked ( and hence exposure) or the subsequent re composed view. Assuming evaluative
    The camera meters at the point you semi-depress the shutter button.
    If you keep the shutter button semi-depressed while you recompose, the camera does not re-meter. Same if you use AE/FE lock (* button).

  5. #25

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    Quote Originally Posted by Diavonex
    mengwei11, I don't know what camera you are using.

    Go to your camera manual and spend some time reading:

    1. Metering mode

    2. Focusing mode

    3. AE Lock

    If you don't get item 1 and 2 sorted out, you'll have plenty of problems getting correct exposure and focus.
    Ok.
    I've read magazines and books on metering and focusing. They keep saying the same thing. I wish they'll have real life step by step examples tho.

    See the other comment. The camera doesn't re meter when you re compose. So in essence metering and focusing are triggered by the half depressing of the shutter button. I know they are different things altogether, but they are both set by the half depress?

    I'm using canon 50d

  6. #26
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    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by mengwei11 View Post
    Ok.
    I've read magazines and books on metering and focusing. They keep saying the same thing. I wish they'll have real life step by step examples tho.

    See the other comment. The camera doesn't re meter when you re compose. So in essence metering and focusing are triggered by the half depressing of the shutter button. I know they are different things altogether, but they are both set by the half depress?

    I'm using canon 50d
    I'm not familiar with the Canon system menu and how to change the settings, but I can jump in with some advice that applies for all systems. By default most cameras have AE set to lock on the scene which you have in your frame when you AF.

    So if you AF with the tree in the middle (so half the frame is dark ground, half the frame is bright sky), your exposure will be locked on the THAT scene, even if you recompose to place the tree at the bottom, and you have pic that is mostly sky and generally overexposed.

    If you fiddle around, you should find out how to have the AE lock not kick in when you AF, it should be somewhere in the camera menus. And AFAIK, the 50D should have an dedicated AE-lock button on the body as well, which allows you to meter for a particular framing with activating AF. I'm not sure how the AE and AF "talk" to each other on a Canon (ie. whether using AF after AE-lock will override the AE-lock), you'll have to figure that one out. Personally I feel that AE-lock is a bit too fiddly, the better alternative is just to switch to M mode, set the right exposure for the scene you want, then just focus and recompose and shoot without having to worry about the exposure at all.
    Last edited by Gengh; 17th February 2011 at 12:14 AM.
    My photos - see just some or all of it =)

  7. #27
    Senior Member Diavonex's Avatar
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    Default Re: New to cs... Evaluative metering

    AE Lock


  8. #28
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    Default Re: New to cs... Evaluative metering

    Maybe you can use nd filter. point at the tree and you can have a nice sky

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    Quote Originally Posted by sachokphantom
    Maybe you can use nd filter. point at the tree and you can have a nice sky
    Usage of ND filter gives you nice skies? How? Could you elaborate?

  10. #30
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    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by sachokphantom View Post
    Maybe you can use nd filter. point at the tree and you can have a nice sky
    Hmmmm, I don't see how that would work. An ND filter reduces the brightness uniformly across the entire scene. If difference in brightness between tree and sky is substantial, you would still either get an overexposed sky or an underexposed tree.

    TS, I think I get what you're asking about the evaluative metering.
    It seems that the camera is metering the scene differently when you half-press the shutter release with the dark object in the centre and at the corner, even though the final image is similar in composition.
    I have also experienced this 'phenomenon' on my camera, which leads me to believe that evaluative metering still has a bias towards the center. Will search for more info and experiment as well.
    Exploring! :)

  11. #31
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    Quote Originally Posted by mengwei11
    Hi, if you set your metering to be evaluative metering, doesn't the camera look at the whole scene and meter? I had very different results tho if I pointed to the sky (bright) and focused vs the tree (dark).

    Help
    You can think of the scene as many segments with different amounts of light. Something like a matrix of holes which you have to fill - exposure in photography has to do with filling holes nicely , half full when desired, full when desired, overflow when desired. As others have explained sometimes you cannot avoid overflowing or underfilling of holes when shooting scenes. At times this can be alleviated somewhat with the use of tools such as graduated neutral density filters (pl google).

    This probably doesn't answer your question directly but understanding exposure is important.

    What do you mean by pointing to the tree and sky? Same scene, different focus point or you shift the camera such that what you see in your frame changes? Would have to understand better to advise. Thx.

  12. #32
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    Quote Originally Posted by mengwei11

    Ok bear with me. But what do u mean by separating metering and focusing?? How do I do that?

    Let's say there's bright sky and u wanted a tree at the lower left corner to be the subject. I use centre focused, lock I'm on the tree and then re compose the tree to be on the lower left corner. Say I set evaluative metering. What does the camera meter?
    It would meter based on first composition. Shutter speed , etc are locked in once you focus.

  13. #33

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    Quote Originally Posted by ZerocoolAstra

    Hmmmm, I don't see how that would work. An ND filter reduces the brightness uniformly across the entire scene. If difference in brightness between tree and sky is substantial, you would still either get an overexposed sky or an underexposed tree.

    TS, I think I get what you're asking about the evaluative metering.
    It seems that the camera is metering the scene differently when you half-press the shutter release with the dark object in the centre and at the corner, even though the final image is similar in composition.
    I have also experienced this 'phenomenon' on my camera, which leads me to believe that evaluative metering still has a bias towards the center. Will search for more info and experiment as well.
    Thanks for understanding. I sometimes wonder if I ask the dumbest questions.

  14. #34
    Senior Member ZerocoolAstra's Avatar
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    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by mengwei11 View Post
    Thanks for understanding. I sometimes wonder if I ask the dumbest questions.


    it seems like the algorithms behind matrix/evaluative metering are quite varied across different manufacturers and even different camera models from same manufacturer.
    No clear indication so far.
    Below is text from Nikon D90 user manual:
    3D color matrix II (metering): Recommended in most situations; selected automatically in auto and scene modes. Camera meters a wide area of the frame and sets exposure according to distribution of brightness, color, distance, and composition for natural results.

    It doesn't explain how WIDE is 'wide', nor does it explain anything about how it calculates the distribution of exposure.
    Exploring! :)

  15. #35
    Moderator ziploc's Avatar
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    Default Re: New to cs... Evaluative metering

    Hi mengwei11,

    In evaluative metering mode (matrix metering in Nikon term), the camera divides the frame into zones and the light intensity at each zone is measured. This data is then compared to the database stored in the camera, to try to guess what scene you're capturing. The final exposure value will be selected based on this evaluation against the database, hence the name "evaluative" metering (or "matrix" which refers to the zones). Older evaluative metering sensors are only monochrome, but almost all newer DSLRs are now using color evaluative metering (Nikon calls it RGB/color matrix metering). By using the color information, the evaluation can now be done more accurately. In contrast, in spot metering the camera only meters at the "spot" defined by spot metering (which is 3.8% at the center of the frame in your 50D).

    Back to your question of the sky & tree, where you use evaluative metering by framing the sky with the tree at one corner. Imagine the frame is divided into zones. The tree is now at one or more zones at the side of the frame (which actually receives less weight), while the sky covers the rest of the zones. In this case, the camera will expose more for the sky since there are more zones for the sky, and some of these zones are at the center. Similarly, if you recompose and let the tree fill the frame, now the tree has more zones including the center ones, so the exposure evaluation will be on the tree. Note that in evaluative metering mode it is not advisable to lock AE when recomposing, simply because the scene is different after recomposing and hence need to be re-evaluated.

    Hope that helps.

    P/S: You can take a look at this KRW article to get a visualisation of the "zoning". The AMP in this article stands for "automatic multi pattern" metering system.
    Last edited by ziploc; 17th February 2011 at 12:16 PM.

  16. #36

    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by mengwei11 View Post
    Hi, if you set your metering to be evaluative metering, doesn't the camera look at the whole scene and meter? I had very different results tho if I pointed to the sky (bright) and focused vs the tree (dark).

    Help
    not sure if i interpret you correctly, but what i understand from your statement is you are at a particular scene, comparing evaluative metering by taking shot with emphasis on tree and sky.

    1. By placing emphasis on tree, you have less sky (bright source).
    2. By placing emphasis on sky, you have more sky.

    So do you think you'll have the same exposure?

    Also do consider that clouds move, thus may have a chance to block the sunlight. which may change the required settings for correct exposure ( exposure f(Aperture, Shutter speed & ISO) altogether.

  17. #37

    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by mengwei11 View Post
    Ok.
    I've read magazines and books on metering and focusing. They keep saying the same thing. I wish they'll have real life step by step examples tho.

    See the other comment. The camera doesn't re meter when you re compose. So in essence metering and focusing are triggered by the half depressing of the shutter button. I know they are different things altogether, but they are both set by the half depress?

    I'm using canon 50d
    When you half press the shutter button, you focus lock.
    When you activate the AE-lock, you lock the exposure.

    They are 2 different thing altogether. unless my memory with the canon system fail.

  18. #38
    Moderator ziploc's Avatar
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    Default Re: New to cs... Evaluative metering

    Quote Originally Posted by chiangkxv View Post
    When you half press the shutter button, you focus lock.
    When you activate the AE-lock, you lock the exposure.

    They are 2 different thing altogether. unless my memory with the canon system fail.
    Yes that's right, the default behavior is that half pressing shutter release button does not lock AE, unless it is specifically set to do so in the camera's settings.

  19. #39

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    Quote Originally Posted by chiangkxv

    When you half press the shutter button, you focus lock.
    When you activate the AE-lock, you lock the exposure.

    They are 2 different thing altogether. unless my memory with the canon system fail.
    Ok, now I'm getting confused. Somebody else posted that when u half press the shutter, u lock the shutter speed and aperture setting (ie exposure) so does it or not?

    If it doesn't like u said, why does the same scene have different exposure when I focus lock on either bright sky or dark tree and recompose to the same scene and the metering is set on evaluative.

  20. #40

    Default Re: New to cs... Evaluative metering

    Pressing half-way the shutter button will trigger the type of light meter set to match the scene.
    It depends what you wish to emphasize whether on tree or sky or not at all. Metering sets the
    correct exposure. Focusing sets which part of the scene you want it to be sharp or everything to
    be equally sharp
    Last edited by henry soh; 17th February 2011 at 01:40 PM.

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